Orthogonal frequency division multiplexing (OFDM) has been used in both wired and wireless communication systems because of its simple implementation and effectiveness in combating inter-symbol interference (ISI). However, OFDM is susceptible to carrier frequency offset (CFO). Even small frequency offsets can cause large signal-to-noise ratio (SNR) and bit-error-rate (BER) degradation. In particular, OFDM systems employing time-domain differential demodulation are very sensitive to the CFO. Therefore, an accurate CFO estimation and correction algorithms should be employed to avoid performance degradation.
The frequency synchronization process can usually be split into an acquisition phase and a tracking phase. The CFO is estimated coarsely and quickly during an acquisition phase, and then a residual or small CFO is estimated more accurately during a tracking phase.
Various CFO correction algorithms have been proposed for OFDM systems. In one approach, a cyclic prefix-based (CPB) algorithm was proposed. The CPB algorithm does not require training symbols or pilot tones. However, the CPB algorithm does not perform well in frequency selective channels since it was designed for an additive white Gaussian noise (AWGN) channel. On the other hand, a pilot tone-aided (PTA) algorithm can estimate the CFO more accurately in frequency selective channels. However, the PTA algorithm requires pilot subcarriers embedded among the data subcarriers, which reduces available bandwidth.